The present invention relates to a data display terminal that is particularly adapted for displaying in sequence across its display screen output data from a remote batch computer which typically contains 132 characters per line.
Conventional data display terminals are limited to a capacity of 80 characters per line. This is due to the limitations inherent in a conventional cathode ray tube. In particular, the screen of a cathode ray tube contains a fixed number of phosphorous dots, and it requires a certain number of dots to provide the necessary resolution to generate a single character. In addition, proper spacing must be provided between each character to make the information readable. Accordingly, the 80 characters per line limitation results.
Due to the limitations on the rastering rate of a cathode ray tube, merely increasing the size of the screen will not increase the effective number of characters which can be displayed per line. Although there has recently been developed high resolution cathode ray tubes which are capable of displaying a full 132 character line, these video monitors are very costly and impose special limitations on the circuitry required for their operation. Consequently, such display monitors have not received a high degree of commercial acceptance.
Thus, it is the primary objective of the present invention to provide a data display terminal using low cost video monitors that is capable of displaying computer print-outs containing 132 characters per line. To accomplish this, the present invention utilizes a pair of conventional cathode ray tubes positioned adjacent one another so that their display screens form a substantially continuous viewing area that is twice the width of a single display screen. The present terminal is adapted to display a line of data in sequence across both screens until the end of the line is reached. In this manner, a full 160 character line capability is attained.
However, output data configurations of computer printouts are typically not all the same. Consequently, in order not to have a column of data interrupted by the separation between the two screens, the present invention includes a "fold control" system that is adapted to effectively shift the block of data either left or right within the 160 character available viewing area. In particular, when displaying a computer print-out having a 132 character line format, the display area of the present terminal has an extra 28 character positions in each line. This column of excess display area allows the data displayed on the screen to be shifted a maximum of 28 character positions until the location of the gap in the viewing area relative to the position of the displayed data is such that the interruption in the viewing area does not seriously detract from its readability. Since the bulk of computer print-outs typically comprise columns of data, it becomes a convenient matter for the viewer to locate the "fold" between adjacent columns. In this manner, the displayed information appears much like the page of a book having two columns of printing.
As will subsequently be explained in greater detail, the operator of the terminal utilizes this feature once data is displayed on the screens simply by depressing a key on the terminal keyboard which shifts the data either left or right as selected one character position per second. When the desired data position is reached, the key is released and the offset position selected is set by the terminal. As additional data is read onto the display screens, the margins will automatically be established by the terminal in accordance with the offset position selected, and the vacant display area on either side of the data will be filled with blanks.
In order to achieve this feature, the effective character address of the data stored in the screen image memory of the terminal must be altered. As will be further described in greater detail, this is accomplished by offsetting the scan character counters for the two CRTs by presetting them to a value determined by the offset established by the terminal operator. In particular, the scan character counter for the left screen memory is preset to a value N counts below zero so that for the first N counts of the scan counter, the screen memory is disabled and only blanks appear on the display screen. After N counts, the scan counter reaches the "zero crossover" point, thereby enabling the screen memory. Thus, the characters stored in the memory begin to appear on the display screen. In a similar manner, the scan counter for the right screen memory is preset to a value N counts below its zero position. Since the "zero" position of the right screen memory corresponds to the 51st character position, the right scan counter is preset to a value of "51 + (28-N)" . Accordingly, the first character to be shown on the right display screen is the character following the last character (79-N) displayed on the left display screen.
To permit the display data to be shifted either left or right across the two screens as described, the memory units for the two display screens must contain overlapping information. Specifically, as will also be described in greater detail, when the information from the computer is entered into the terminal memory, the column of information from the 51st character position to the 79th character position in each line is stored in the screen memory of each video monitor. This is to insure that the remaining characters in the 132 character line not displayed on the left screen will appear on the right screen. Accordingly, as will subsequently become more apparent, once the desired offset position is selected by the terminal operator, the fold control circuitry of the present invention automatically effects the display of the first character in the 132 character line on the left screen indented from the left margin by the number of character positions corresponding to the selected offset value, followed by as many succeeding characters as will fit on the first line of the remaining display area of the left display screen; and the display of the balance of the 132 characters not displayed on the left screen on the right screen with blanks comprising the remaining character positions in the first line of the right display screen. This process is repeated for each of the 32 lines of the display area. In this manner, information displayed on the terminal is read in sequence across both display screens irrespective of the positioning of the data within the total display area.
Additionally, the preferred embodiment of the present invention utilizes a pair of random access memory units for the screen image memory of the display terminal, instead of conventional shift registers, so that the effective memory capacity of the terminal is increased. In particular, the RAMs employed are preferrably selected to be large enough to store up to four computer pages of information; a computer page being customarily defined as having having 132 characters per line and from 60-64 lines. In other words, the preferred memory capacity of the display terminal can accommodate eight times as much data as can be displayed on the display screens at any one time. In addition to providing the terminal operator with immediate access to a greater amount of information, the increased memory capacity also facilitates the transmission of multiple screens of information at a single time which improves the efficiency of batch communication.
The present invention also includes a novel heading protect circuit which isolates a designated number of lines at the top of the display screen from the balance of the display area which is subject to being shifted either up or down under the control of the scroll circuit. The heading protect circuit functions by overriding the line address during the read mode of operation for the designated number of lines after which the line address is automatically shifted to the location of the data desired to be displayed.
Finally, the present invention includes a unique message page feature which permits batch computer questions, corrections, or error messages to be stored separate from the working text area of the terminal memory. This permits the present terminal to receive and store communications from an external data processor without sacrificing part of the working text area of the terminal memory. Thus, for example, when a terminal operator commits an error, rather than overwriting the error message on the display screen and destroying the text being prepared, the present display terminal is adapted to signal the operator by flashing a light mounted on the keyboard console. At the same time, the error message from the batch computer is automatically entered into the separate "message page" area of the terminal memory. The operator then merely depresses the appropriate key on the keyboard to display the error message. When the key is released, the error message is removed and the text previously prepared by the operator can be retrieved intact. Thus, the appropriate corrective editing can easily be performed without having to rebuild the entire text or a substantial part thereof.